1. Technical Field
The present invention relates generally to thermal cores. More particularly, it relates to a method of making a thermal core material and the material so made.
2. Related Art
Thermal cores are devices which are used to mount electronic circuit boards and to aid in conducting heat away from the circuit boards. In the related art, layered carbon fiber laminates, i.e., prepregs, have been used in thermal cores because of the high thermal conductivity of carbon. Further, the high thermal conductivity carbon fibers, such as P100, P120, K1100 manufactured by Amoco Advanced Composites Corp., offer more lightweight high stiffness than traditional aluminum cores. However, while these carbon fibers all have exceptionally high axial thermal conductivity, they exhibit very low transverse conductivity, i.e., transverse to the length of the fiber. Accordingly, when these fibers are used in a composite laminate in layers, the in plane thermal conductivity is very high but the transverse thickness conductivity is very low and usually limited by the resin that is used to bond the carbon fibers together.
The following related art, which is hereby incorporated by reference, have attempted to increase the transverse conductivity in a number of ways. For instance, as illustrated by U.S. Pat. No. 5,255,738 to Przilas and assigned to E-Systems, Inc., it is known to bond a metal cap to the edge of the core at the card guide interface to improve the heat transfer from side to side. Further, as exemplified by U.S. Pat. No. 4,975,261 to Takabatake and assigned to Petoca, Ltd., it is known to load a liquid with a carbonaceous material and then immerse a bundle of carbon fiber therein during manufacture of the prepreg. Additionally, as illustrated by U.S. Pat. No. 4,902,453 to Okura et al. and assigned to Across Co., Ltd., it is known to provide a mixed powder including finely divided carbonaceous binder pitch and finely divided coke between reinforcing fibers of a composite, i.e., during prepreg manufacture.
The above references, however, add complexity and cost to the manufacturing process, and weight to the thermal core. In particular, the addition of a metal cap is expensive, and adds weight and an additional thermal interface that presents long term reliability issues because of the bond. The addition of carbon material to resins prior to immersion of the fiber layers therein results in an unacceptable loss of strength and an unfocused dispersion of thermally conductive particles in the composite. Further, addition of material to the resin may contaminate other aspects of the prepreg manufacturing process.
Accordingly, there has been a need for a carbon fiber thermal core manufacturing process which increases transverse conductivity of a fiber thermal core, but maintains manufacturing costs, complexity and low weight.